1. Technical Field of the Invention
The present invention relates generally to a spark plug with increased durability and carbon fouling resistance for internal combustion engines which may be used in automotive vehicles, co-generation systems, or gas feed pumps.
2. Background Art
Japanese Patent No. 3140006 (U.S. Pat. No. 6,229,253 B1) teaches a multi-ground electrode spark plug for internal combustion engines. FIG. 22 shows a multi-ground electrode spark plug 9 of the same type.
The spark plug 9 includes a porcelain insulator 92, a center electrode 93 retained within the porcelain insulator 92, a metal shell 94 in which the porcelain insulator 92 is retained with an insulator head 921 exposed outside the metal shell 94, and a main ground electrode 951 defining a main spark gap 911 between itself and a tip 931 of the center electrode 93.
When the combustion temperature is extremely low in the engine, so that the temperature of the surface of the porcelain insulator 92 is hardly increased, it may cause the engine to smolder, so that a layer of carbon is deposited on the porcelain insulator 92, thereby resulting in a drop in insulation resistance between the center electrode 93 and the metal shell 94, which, in the worst case, leads to misfiring of the engine.
In order to avoid the above problem, the spark plug 9 also includes auxiliary ground electrodes 952 which are welded to the metal shell 94 and face the side surface of the center electrode 93 through the insulator nose 921 to define auxiliary spark gaps 912. When the carbon is deposited on the porcelain insulator 92, so that the insulation resistance between the center electrode 93 and the metal shell 94 drops, sparks are produced within the auxiliary spark gaps 912 to burn off the carbon deposit to clean up the surface of the porcelain insulator 92.
The spark plug 9 is so designed as to induce discharge of sparks in the auxiliary spark gaps 912 only when the engine is smoldering, so that a layer of carbon is deposited on the porcelain insulator 92 and to discharge sparks mostly in the main spark gap, thereby eliminating channeling (i.e., formation of channels, as illustrated in FIGS. 10 and 11, in the surface of the porcelain insulator 92 caused by the discharge of sparks in the auxiliary spark gaps 912) and minimizing the wear of the center electrode 93, as illustrated in FIG. 8, to enhance anti-fouling characteristics (i.e., carbon fouling resistance) and service life of the spark plug 9.
However, in modern internal combustion engines, an increase in compression ratio, supercharging, lean-burning, or an increase in amount of EGR results in an increased flow velocity of mixture within combustion chambers of the engine, which biases sparks toward the auxiliary spark gaps 912 to increase the rate of occurrence of sparks in the auxiliary spark gaps 912. This causes sparks to be discharged within the auxiliary spark gaps 912 even when the engine is not smoldering, thus accelerating the channeling and wear of the center electrode 93, which leads to a greatly decrease in service life of the spark plug 9.
Each of the auxiliary ground electrodes 952 structurally induces a strong electrical field at an edge 954 of an end face 953 closer to the metal shell 94, which may cause sparks to be discharged in the auxiliary spark gap 912 frequently when the porcelain insulator 92 is not fouled with carbon, thus accelerating the channeling and the wear of the center electrode 93.
The auxiliary ground electrode 952 may be shaped not to induce the strong electrical field. This, however, results in a decrease in spark in the auxiliary spark gaps 912 when the engine is smoldering, thus giving rise to a deterioration of carbon fouling resistance of the spark plug 9.